1. Field of the Invention
The present invention relates to an all-optical light modulating apparatus such as optical modulators and optical switches and a process for modulating light capable of speedily modulating light by an all-optical means, which are suitable to be used in optical communication systems and the like.
2. Related Background Art
In recent years, several methods of converting information contained in a shorter wavelength (e.g., 800 nm) light beam to that contained in a light beam of a longer wavelength (e.g., 1.55 .mu.m) have been proposed. One is to let the shorter wavelength light be detected by a photodetector which is in turn connected to an electronic control circuit. The circuit modulates a longer wavelength laser electrically by either a means of direct current modulation or an external electrooptic modulator utilizing phase shift or electro-absorption. In the former, the longer wavelength laser is directly oscillated by modulating a driving current thereof. In the latter, the longer wavelength laser is continuously oscillated by a constant driving current, and the thus oscillated light beam is modulated by the external modulator using an electrooptic crystal or the like.
In such a structure, even though the photodetector, the control logic, the modulator and/or laser can be fully integrated on a single substrate, there are some obstacles for doing so. One is that lasers or modulators working at 1.55 .mu.m are based on other material systems (i.e., InGaAs, InSb and the like) than those working at 800 nm (i.e., AlGaAs and the like), so that the former devices are not completely problem-free to integrate with the 800 nm components which are based on the AlGaAs material systems. Working with lasers or modulators in the 1.55 .mu.m material systems implies a problem of high absorption due to the closeness of band gaps of those materials. Further, material incompatibility (lattice mismatch) implies manufacturing obstacles and a degradation of the crystal structure which in turn result in unnecessary scattering of the light subject to modulation.
When using a directly modulated 1.55 .mu.m laser, the modulation speed is most certainly limited by chirp or other unwanted transient effects of directly modulated lasers.
In the case of operating a 1.55 .mu.m CW (continuous wave) laser and employing an external modulator, the modulation speed is limited by the RC time constant of the modulating device, as in the case of direct laser modulation.
This is in fact a problem that one always has to cope with when considering the electrooptic alternative for controlling a 1.55 .mu.m beam by a 800 nm beam.
Conventional all-optical and semi-optical alternatives lack this drawback. Those previous optical modulators, however, have not been dealing with the control of a beam of a wavelength considerably different from that of the controlling beam, but rather signal beams of the same wavelength range. One example is the SEED (SElf Electrooptic Device) which is based on the voltage drop induced by a photo-current flowing through an external resistance serially connected with the device. The flow of photo-current is caused by a light beam incident on the device. The voltage drop in turn changes the inclination of a band energy structure of the device to shift its absorption edge, so that another light beam or the incident light beam itself can be intensity-modulated. However, a 1.55 .mu.m beam cannot be intensity-modulated by a 800 nm beam using the SEED device. In conclusion, even though it is possible to utilize the change in the voltage to induce a refractive index change, there are severe problems of large RC time constants because of the use of external resistor circuits.
Further, there exists, as a scheme that allows an intensity modulation, a device in which the index grating is optically induced to in turn deflect a beam subject to modulation. In this case, in order to obtain a large on/off contrast ratio, the diffraction efficiency of the grating must be of a considerable magnitude. In order to achieve this, either light beams having large intensities or ferroelectric materials having a large electrooptic coefficient, should be used. However, the use of available ferroelectric materials implies a very slow response of the order of a second or slower, which, of course, is completely undesirable in optical communication systems.
Furthermore, there exists a device utilizing a photorefractive effect (PR effect). The PR effect signifies the following phenomenon. When an electron located in a given impurity level is optically excited and moved to a conduction band, the electron is diffused and drifted in the conduction band and then captured in a capture level. A spatial charge electric field generated by the captured electron results in a nonleinear change in refractive index due to an electrooptic effect (i.e., a Pockels effect). The impurity level is generally called a deep level having an energy gap sufficiently larger than that of thermal energy (26 meV) at a room temperature, in contrast to a donor or acceptor level serving as a shallow level. The capture level is generally an ionization level of the impurity level.
By using photorefractive semiconductors, the response time becomes considerably faster, but unfortunately, the grating efficiency will be much lower resulting in a low-contrast all-optical device. Another drawback of the index grating-based devices using the photorefractive index crystal is that they require two coherent beams to write the grating therein. Since the information-bearing light at 800 nm is most oftenly confined in integrated waveguides, it is troublesome to interface devices containing those waveguides with the two-beam index grating device.
Recently, a new photorefractive effect which does not require a grating to be written by two coherent beams has been discovered. A change in the refractive index of a graded-gap quantum well structure (the structure in which the band gap changes along a direction of the barrier layer) is induced by the carrier generation due to an optical field. The carriers migrate due to an internal electric field caused by the graded gap to finally be trapped in quantum wells. Also the corresponding holes are subject to this process, but at a slower rate, so that there will be a spatial charge separation between electrons and holes resulting in a transient space-charge electric field. As a result, the refractive index is changed via the electrooptic effect. In this connection, see "Ralph et. al., Physical Review Letters, vol. 63, pp. 2272-2275 (1989)". In such a device, although the disadvantage of relying on two beams to induce an index change which could be probed by a beam subject to modulation has been eliminated, there is still a major disadvatage left, namely, a slow response time resulting from the limited migration from the location of excitation to the wells where the carriers are trapped. The response time is thus dependent both on this fact and the recombination time in the wells.